For many years, it has been possible to locate the slag surface in a metallurgical vessel, if it is stable, by using microwave, laser, eddy-current, radioactive, camera or float technology. It has also been possible to roughly measure the extent of different zones in the vessel by employing the use of a simple dip pin/sounding bar technology where a metal rod attached to a chain or other delivery system is first immersed in the molten material and then withdrawn for visual inspection. An experienced operator may thereby visually estimate the location of a particular zone in the vessel at that moment in time.
There are also electronic measurements devices designed for measuring the thickness or other properties of the slag layer. U.S. Pat. No. 5,781,008 discloses a measurement device which uses a combination of sensors attached to a moveable lance. One sensor is arranged at the tip of the lance to sense contact with the slag layer and another sensor is configured to remotely (via eddy current) sense the distance to the interface between the slag layer and the molten metal. As the tip contacts the slag layer, the eddy current sensor is operated to determine the distance to the slag/melt interface. Since the distance between the sensors is known, the slag layer thickness can be determined.
U.S. Pat. No. 4,647,854 and JP06-258129 suggest the use of an eddy current-type distance measuring sensor suspended above the slag layer in a metallurgical vessel for detecting the level of molten metal in the vessel. The sensor comprises an excitation coil for generating an oscillating electromagnetic field and one or more eddy current detecting coils.
U.S. Pat. No. 4,841,770, US2007176334A1, JP2003049215 and U.S. Pat. No. 4,880,212 all disclose different moveable lances with measurement devices (probes) configured to be immersed into the slag layer so as to generate a signal indicative of the slag thickness. The sensors and sensor electronics are designed to sense the interface between the slag layer and molten metal, e.g. using electrode pairs, or inductive coils connected to oscillators. Thus, the sensors and the sensor electronics are designed to provide a well-defined switch point at the interface between the slag layer and the molten metal.
DE3201799 discloses use of electrodes for measuring the conductivity of a slag layer.
JP1094201 describes a technique for measuring the thickness of molten slag, by arranging a magnetic field generation coil and a pair of detection coils above the slag, and by driving the coils such that a resistance component indicative of the thickness may be isolated by an impedance measuring device.
US2007/173117 discloses a design of a measuring head for attachment to a lance, the measuring head including a temperature sensor and an oxygen sensor for measuring a corresponding parameter of the slag layer or the molten metal beneath the slag layer.
U.S. Pat. No. 5,198,749 discloses an sample probe operable to suck molten metal through an orifice for measuring the number and size of non-conductive inclusion particles.
U.S. Pat. No. 5,827,474 discloses a technique for measuring the depth of molten steel and slag in a metallurgical vessel. A probe of electrically conductive material has a proximal end electrically connected to a voltmeter, and a distal end movable between the vessel floor and the air-slag interface or the slag-steel interface in the vessel. The distal end thus acts as an electrode, and the depth of molten steel or the depth of the slag is determined by comparing the differences in electrical potential detected by the voltmeter while noting the vertical position of the distal end of the probe.
JP11104797 discloses a technique for avoiding outflow of molten slag from a ladle during tapping of molten metal into a tundish. The technique involves comparing the electrical conductivities measured by a pair of electrodes in a bottom part of the ladle and by a pair of electrodes in the tundish. A deviation in measured conductivity between the ladle and the tundish is taken as an indication that the molten slag has reached the electrodes in the ladle.
U.S. Pat. No. 4,150,974 discloses a technique for positioning the snorkel of a vacuum degassing apparatus beneath the interface of the molten metal and the slag in a ladle. The location of the interface is determined by vertically displacing an electrode which is in electrical contact with the material in the ladle. The position of the metal-slag interface is determined by noting a change in the voltage produced by the electrode.
WO2009/109931 discloses a probe for use in control of a solvent exchange process. Along its extent, the probe carries a series of pairs of sensing pins for measuring resistance. By immersing the probe into the material subjected to the exchange process, a resistivity profile along the length of the probe may be determined.
In some melting/refining processes, the vessel contains a number of material layers, as well as areas of gradual change or material mixing. For example, in processes for melting copper or platinum, it is known that there is a large mixing zone between the slag layer and the matte. At present, there is no versatile technique for probing of any part of the target material in a metallurgical vessel, e.g. for the purpose of analysing the presence and/or location of different zones/layers, such as material layers and mixing zones located beneath a slag layer in the vessel. Such a technique would have great value, e.g. to decision making and process optimization.
To the extent that the techniques proposed in aforesaid U.S. Pat. No. 5,827,474, JP11104797, U.S. Pat. No. 4,150,974 and WO2009/109931 may be applied for such use, these techniques all rely on probes with electrodes/sensing pins that must be in direct galvanic contact with the target material in the metallurgical vessel. Such probes will have an elevated sensitivity to deposits and contaminations, as well as a potentially reduced life since the electrodes/sensing pins are directly exposed to the harsh environments in the vessel.